1. Field of the Invention
The present invention relates to a thin-film transistor that is used in such devices as liquid crystal display devices.
2. Description of the Related Art
A major reason for forming thin-film transistors on translucent substrates composed of materials such as glass or quartz is to realize optically transparent optical devices. An active matrix liquid crystal display device is a representative example of an optically transparent optical device in which thin-film transistors are used in the control of display pixels. Liquid crystal display devices are used in various types of portable equipment such as personal computers, mobile telephones, or personal digital assistants (PDAs), and more recently, in thin-screen television image receivers. In these types of equipment, direct-view liquid crystal display devices are used in which the image that is displayed on a panel is viewed directly by the unaided eye. Liquid crystal display devices are also employed in the enlargement/projection optics of projectors for projecting an image onto a screen.
A liquid crystal display device (liquid crystal light valve) that is used in a projector typically irradiates a far more intense light than a direct-view liquid crystal display device. For example, if a type-1 screen that is enlarged to the equivalent of a type-100 screen projects an image onto a screen having the same level of brightness as a type-15 direct-view liquid crystal display device, a conversion based on area ratio shows that the amount of light that is irradiated onto a liquid crystal light valve is more 2,000,000 times brighter per unit area than the light irradiated onto the direct-view type. As a result, there is the problem that in an active matrix liquid crystal display device that is used as a liquid crystal light valve, carrier is generated in thin-film transistors due to photoexcitation, thereby increasing leak current (optical leak current) to a high level.
In an active matrix liquid crystal display device, a thin-film transistor is normally turned ON to apply a desired voltage (write voltage) to a pixel electrode, following which the write voltage must be sustained until the next write operation by turning the thin-film transistor OFF. When the optical leak current is great, however, the write voltage that is being maintained will drop, and the screen contrast therefore will also drop.
In order to suppress the optical leak current, instead of forming the active layer directly on a translucent substrate with an underlayer insulating film interposed, the active layer (also referred to as “islands” because it is formed in island form) of a thin-film transistor may be formed after first using a light-blocking material (such as a metal material) to form a light-shield film on an underlayer insulating film, stacking an additional underlayer insulating film, and then forming the semiconductor layer that is the active layer over this underlayer insulating film. In other words, light that is incident from the direction of the translucent substrate may be blocked by means of a light-shield film that is arranged between the active layer and the translucent substrate.
Explanation of the Planar Layout
As shown in FIG. 1, the active matrix liquid crystal display device that is used as a liquid crystal light valve is a construction in which gate electrodes 7 and data electrodes 10 are each arranged in a matrix form such that gate electrodes 7 and data electrodes 10 are orthogonal to each other, and pixel electrodes 18 that are composed of ITO (Indium-Tin Oxide) are arranged in each of the display areas that are partitioned by gate electrodes 7 and data electrodes 10. Thin-film transistors (TFT) for applying write voltage to pixel electrodes 18 are formed in the regions at which gate electrodes 7 and data electrodes 10 intersect.
FIG. 2 is an enlarged view of the region that is enclosed within the oval in FIG. 1 and shows an example of the configuration of the TFT formation region.
As shown in FIG. 2, data electrode 10 is connected to the source of the TFT by way of data electrode TFT contact 16, and drain electrode 8 is connected to the drain of the TFT, which is linked to pixel electrode 18, by way of ITO-TFT contact 17.
The source and the drain of the TFT are each formed in respective semiconductor layers (not shown), and a channel (a region that is covered by gate electrode 7) is formed between the source and drain. In addition, lightly-doped drain (LDD) regions 15 having an impurity concentration that differs from that of the source and drain are formed on the channel sides of each of the source and drain.
Explanation of the Sectional Views
FIG. 3A is a side sectional view showing the appearance of the cut surface as seen from line C-C′ of the liquid crystal display device that is shown in FIG. 2, and FIG. 3B is a side sectional view showing the appearance of the cut surface as seen from line D-D′ of the liquid crystal display device that is shown in FIG. 2.
As shown in FIGS. 3A and 3B, the liquid crystal display device includes translucent insulation substrate 1 that is composed of a material such as glass, and lower light-shield film 3 is formed on this translucent insulating substrate 1 with underlayer insulating film 2 interposed, and a TFT is formed on first interlayer film 4 that is formed so as to cover lower light-shield film 3.
The TFT is a construction that includes semiconductor layer (polysilicon) 5 in which the source/drain, LDD regions 15, and channel are formed; gate insulating film 6 that is formed on semiconductor layer 5; and gate electrode (see FIG. 3B) 7 that is formed on gate insulating film 6. Data electrode 10 is formed on gate electrode 7 with second interlayer film 9 interposed.
Further, data electrode 10 is formed on second interlayer film 9, third interlayer film 11 is formed so as to cover data electrode 10, and black matrix 12 is formed on third interlayer film 11. On black matrix 12, a common substrate is arranged with a liquid crystal layer interposed (neither being shown).
Black matrix 12 blocks light that is incident from the direction of the opposing common substrate that sandwiches the liquid crystal layer. On the other hand, lower light-shield film 3 blocks light that is incident from the direction of translucent insulating substrate 1 (in the case of a projector, reflected light from the optics).
Explanation of the Black Matrix
Black matrix 12 is in some cases formed within the same substrate as the TFT as shown in FIGS. 3A and 3B, and in some cases is formed within the common substrate that opposes the TFT with the liquid crystal layer interposed.
When black matrix 12 is formed in the common substrate, a positional shift of approximately 10 μm that occurs in the process of stacking the two substrates must be taken into consideration, and black matrix 12 must consequently be formed larger than lower light-shield film 3. As a result, the problem occurs that the open area ratio of the pixels cannot be increased.
Accordingly, the configuration shown in FIGS. 3A and 3B in which black matrix 12 is formed in the same substrate as the TFT is frequently adopted. In this type of configuration, a high level of alignment accuracy can be obtained by taking advantage of the fabrication process of the semiconductor device, and the large misregistration that occurs in the above-described process of stacking the two substrates therefore need not be considered. However, an adequate countermeasure has not been found for blocking the diffuse reflection of light that also occurs within the substrates such as shown in FIGS. 3A and 3B.
Light that is incident from the direction of the common substrate or light that is incident from the direction of translucent insulating substrate 1 is not made up of only components that are parallel to the gate electrode, but includes components of various directions, and there is consequently the concern that the incident light will reach the semiconductor layer that underlies the gate electrode.
For example, in the region in which gate electrode 7 is formed as shown in FIG. 3B, light that is incident from the direction of the common substrate or light that is incident to translucent insulating substrate 1 is adequately blocked by lower light-shield film 3 and black matrix 12.
However, in regions that lack gate electrode 7 as shown in FIG. 3A, the widths of lower light-shield film 3 and black matrix 12 are both typically limited to raise the open area ratio of the pixels. Thus, in these regions that lack gate electrode 7, the width of black matrix 12 is normally set in accordance with the width of lower light-shield film 3 such that light that is incident from the direction of the common substrate does not reflect from the surface of lower light-shield film 3 and reach the semiconductor layer of the TFT. In contrast, light that is incident from the direction of translucent insulating substrate 1 undergoes multiple reflections, for example, between black matrix 12 and lower light-shield film 3 or between data electrode 10 and lower light-shield film 3, and thus reaches the semiconductor layer of the TFT.
A method has consequently been proposed in which a prescribed dc voltage is applied to lower light-shield film 3 as a method for reducing the optical leak current in the above-described TFT (for example, refer to Japanese Patent Laid-Open Publication No. H10-111520). In Japanese Patent Laid-Open Publication No. H10-111520, a substantial reduction of the optical leak current is achieved by applying an optimal dc voltage to lower light-shield film 3 for each TFT.
However, in a liquid crystal display device in which a multiplicity of TFTs is arranged, a common voltage (in the following explanation, this voltage is referred to as the “back-gate voltage”) is normally applied to the lower light-shield film 3 of each TFT. Thus, when the optimum value of the voltage that is to be applied to each lower light-shield film 3 varies for each TFT, the back-gate voltage must be set to within an extremely narrow range in order to suppress the leak current (including the optical leak current) of all TFTs to a desired value or less.
The leak current in a TFT in which the back-gate voltage diverges from the optimum value increases markedly and therefore produces display defects in the corresponding pixels of the liquid crystal display device and lowers reliability. In addition, accurately setting the back-gate voltage to a desired voltage necessitates the use of an expensive voltage generation circuit, which raises the additional problem of increased fabrication cost.